Here's an extract of the complete install guide. I use php5.external where you want to use php-cgi.

# phpfpm/fastcgi
# Here we catch the 'false' Location used to inexistent php5.external
# and push it to the external FastCgi process via a socket
# note: socket path is relative to FastCgiIpcDir
# which is set in Main configuration /etc/apache2/mods-available/fastcgi.conf
<IfModule mod_fastcgi.c>
# all .php files will be pushed to a php5-fcgi handler
AddHandler php5-fcgi .php
#action module will let us run a cgi script based on handler php5-fcgi
Action php5-fcgi /fcgi-bin/php5.external
# and we add an Alias to the fcgi location
Alias /fcgi-bin/php5.external /php5.external
# now we catch this cgi script which in fact does not exists on filesystem
# we catch it on the url (Location)
<Location /fcgi-bin/php5.external>
# here we prevent direct access to this Location url,
# env=REDIRECT_STATUS will let us use this fcgi-bin url
# only after an internal redirect (by Action upper)
Order Deny,Allow
Deny from All
Allow from env=REDIRECT_STATUS
</Location>
</IfModule>
FastCgiExternalServer /php5.external -socket myapplication.sock -appConnTimeout 30 -idle-timeout 60